Papers by Author: Jai Sung Lee

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Authors: In Hyung Moon, Yu-Chan Kim, Sung Tag Oh, Jai Sung Lee
Authors: Jai Sung Lee, B.H. Cha, H.G. Kang, Yun Sung Kang
Abstract: This paper overviews our recent investigations on the processing of net-shaped Fe-based nanoparticulate materials and their related material properties such as mechanical and corrosion properties. The key-process for fabricating fully densified net-shaped nanopowder by pressureless sintering is an optimal control of agglomerate size of nanopowder. Enhanced mechanical property of powder injection molded Fe-Ni nanopowder could be explained by grain refinement and uniformity of microstructure.
Authors: Sergiy V. Divinski, Frank Hisker, Yun Sung Kang, Jai Sung Lee, Christian Herzig
Authors: H.Y. Nam, S.K. Kwon, Young Soo Kang, Jai Sung Lee
Abstract: Sintering process of powder injection molded (PIMed) Fe-50wt%Ni nanoalloyed powder was investigated in association with microstructure development and residual impurity effect. Compared to conventional powder metallurgical (PM) processed Fe-Ni nanoalloy powder, the PIM compact showed a homogeneous and uniform densification behavior. This is owing to more homogeneous particle distribution in the PIM resulting from preparation of feedstock which was fabricated by mixing of nano powder with thermoplastic binder. Residual impurities originating from the binder material did not have any apparent influences on sintering behavior. Conclusively, Fe-50wt%Ni nanoalloy powder is effectively applicable to the PIM parts.
Authors: Hyoung Seop Kim, Jai Sung Lee
Abstract: A phase mixture model (PMM) was considered in which materials are treated as a mixture of grain interior phase, grain boundary phase and pores (if the material is porous) for the elasticity and plasticity of nanostructured materials (NSMs). In order to investigate the effects of grain size and porosity on the elastic modulus, a self-consistent method in conjunction with PMM was employed. The calculated results are compared with the experimental measurements in the literature. The elastic modulus of NSMs decreases with a decrease of the grain size and the decrement is relatively large at grain sizes below about 10 nm. The effect of porosity, however, is substantially greater than the grain size effect. For the plasticity of NSMs, grain size effects were introduced both via the dislocation glide mechanism and through the diffusion mechanisms providing mass transfer via grain boundaries. A good agreement between the calculated deformation behavior and experiment was found. The quality of the above predictions with regard to strength, strain hardening, strain sensitivity and ductility behavior testify the adequacy of the model. It is concluded that the model can be used as a convenient tool for simulating the deformation behavior of NSMs.
Authors: C.W. Lee, S.G. Kim, Jai Sung Lee
Abstract: The influence of reaction temperature on phase evolution of iron oxide hollow nanoparticles during chemical vapor condensation (CVC) process using iron acetylacetonate was investigated. X-ray diffraction (XRD) analyses revealed that three iron oxide phases (α-Fe2O3, γ-Fe2O3, and Fe3O4) and a mixture of β-Fe2O3 and small amount of γ-Fe2O3 were synthesized at 700oC and 900oC, respectively. TEM observation disclosed that the iron oxide particles are almost composed of hollow structured nanoparticles of 10~20 nm in size and 3~5 nm in shell thickness. This result implies that reaction temperature determining various reaction parameters plays an important role for the phase- and structural evolutions of iron oxide hollow nanoparticles. Especially, the present investigation attempted to explain temperature dependence of the phase evolution of β-Fe2O3 hollow nanoparticles in association with the decomposition of iron acetylacetonate.
Authors: Ki Hun Seong, Jai Sung Lee
Abstract: Synthesis of iron nanopowder by room-temperature electrochemical reduction process of α-Fe2O3 nanopowder was investigated in terms of phase evolution and microstructure. As process variables, reduction time and applied voltage were changed in the range of 1~20 h and 30~40 V, respectively. From XRD analyses, it was found that volume of Fe phase increased with increasing reduction time and applied voltage, respectively. The crystallite size of Fe phase in all powder samples was less than 30 nm, implying that particle growth was inhibited by the reaction at room temperature. Based on the distinct equilibrium shape of crystalline particle, phase composition of nanoparticles was identified by TEM observation.
Authors: Kyung Jong Lee, Jai Sung Lee
Abstract: This work has attempted to find a new low temperature reduction process for fabrication of Cu nanopowder from fine CuO powder. For this purpose, we used electrochemical reduction method which is conducted in an electrolyte of NaCl aqueous solution at room temperature. It was found that ball-milled CuO powder (particle size ~100 nm and grain size ~40 nm) was completely reduced under the conditions of 20 V power, 0.5 mol NaCl solution and 2 h reaction time, producing Cu nanopowder (particle size ~80 nm and crystallite size ~25 nm). Simultaneously, we observed that sintering of nanopowders occurred during the reduction process, leading to agglomeration of nanopowder. Based upon the experimental results, the correlation between electrochemical reduction process and its related powder characteristics was discussed in terms of material transport.
Authors: Jung Goo Lee, J.C. Yun, Jai Sung Lee, C.J. Choi
Abstract: Calciothermic reduction-diffusion (CRD) method was employed to prepare the Sm2Fe17 powder. By using CRD method, single-phase Sm2Fe17 powders were successfully made and no α-Fe phase detected. And Subsequent suitable nitrogenation treatment after CRD process enabled us to obtain Sm2Fe17Nx magnetic powders. However, the magnetic performance of the powders was below expectation due to their large particle size. Further study on effective milling process is needed.
Authors: S.S. Jung, Yun Sung Kang, Jai Sung Lee
Abstract: The present investigation has attempted to optimize hydrogen reduction process for the mass production of Fe-8wt%Ni nanoalloy powder from Fe2O3-NiO powder. In-situ hygrometry study was performed to monitor the reduction behavior in real time through measurement of water vapor outflowing rate. It was found that the reduction process can be optimized by taking into account the apparent influence of water vapor trap in the reactor on reduction kinetics which strongly depends on gas flow rate, reactor volume and reduction.
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